(1) Field of the Invention
The present invention is directed to a linear antenna for dual frequencies and a method for designing such an antenna and more particularly to a twin-axial floating antenna that can be deployed from a vessel.
(2) Description of the Prior Art
Previous work on buoyant cable antenna (BCA) improvements have led to antennas that have improved performance in the HF band (e.g., U.S. Pat. No. 7,868,833 entitled “Ultra wideband buoyant cable antenna element”) but this improvement came at the expense of the performance of the antenna in the VLF/LF band (10 kHz-200 kHz. This occurred because the methods involved required the use of series capacitive loading along the length of the antenna wire. This creates a high pass filter in the antenna and prevents current flow in the VLF/LF bands.
U.S. Pat. No. 8,203,495, entitled “Modular VLF/LF and HF buoyant cable antenna and method” introduces a useful method of including the VLF/LF capability back into the antenna. In this patent, the VLF/LF signals are received on the braid of a piece of coaxial cable that is connected in series with the loaded HF antenna. However, this method suffers from two major shortfalls. The first is that the attenuation of the coaxial cable hinders the gain of the HF antenna, since HF signals must pass through the coaxial cable to reach the overall antenna feed point. The second is that the overall antenna length increases to over 150 feet long, which is undesirable from a mechanical point of view since it affects the speed-depth curves and hinders the submarine's operations when using the antenna.